Standalone STI

Move STI to standalone repository
- Add ncdc's work to stream tars
- Refactor code and add unit tests
- Switch to using Godeps
Wasn't sure about this one. It makes sense to abstract fs stuff in the higher level code (Builder/Executor). But here I'm not sure I get much benefit. If I want to test it, I really have to deal with the filesystem directly. (I tried abstracting things, and couldn't come up with something that would allow me to test without a real file system). If you see the tests for tar, they actually work with real tar files and directories created by ioutil. If you have suggestions on a way to avoid that, I'm all for it.